The invention relates generally to the field of injection molding. More particularly, the invention concerns an injection molding method having a pressure relief valve that provides an alternate resin passage for excess molten resin under a predetermined pressure in a non-metallic injection mold.
Cost reductions in low to mid volume injection molding operations has led to the use of one piece case epoxy molds as an alternative to more expensive steel or aluminum tooling. Several have developed techniques to produce production injection molded parts using these cast epoxy molds. The epoxy molds are cast as a single piece for each half of the mold as opposed to hard tooling that is machined out of steel or aluminum. The main limiting factor of the epoxy mold is its inability to withstand the normal cavity pressures created within a conventional mold. Epoxy molds will only tolerate 10%-15% of the pressure that a steel or aluminum mold will tolerate. But they still require adequate pressure during injection and packing to produce an acceptable part. If cavity pressure should spike during injection or pack, the epoxy mold will fail catastrophically. This means that one half, or in some cases both halves, of the mold will split into pieces, effectively destroying the mold.
It is our experience that epoxy injection molds for molding low to mid volume production parts need protection against catastrophic failure. The method and apparatus for protecting these molds needs to be simple, adjustable for different molding resins, specific to each mold so that the mold is protected in any molding machine, and the method and apparatus needs to reset itself automatically whenever it is actuated to protect the mold. Several known attempts have been made to resolve these problems unique to non-metallic molds but most were rejected because of cost and the problem of modifying all the different molding machines that these molds can run in.
In U.S. Pat. No. 5,350,288, Sep. 27, 1994, by Kimoto et al, entitled Injection Molding Unit, an injection molding unit is disclosed that uses a pressure relief valve in a conventional metallic (steel) mold. A major shortcoming of the aforementioned valve arrangement is that if the relief valve operates and releases excess melt pressure from the mold, the mold and/or relief valve assembly will have to be disassembled to remove the solidified plastic material. Moreover, the relief valve operates as a part of the injection molding machine without any obvious means of adjusting the pressure.
Therefore, a need persists in the art for an injection molding method having a non-metallic mold that resists catastrophic failure under high molten resin pressure.
An object of the present invention is to provide a method of releasing pressure in an injection molding apparatus having a non-metallic injection mold for molding low to medium volume production parts without catastrophic failures.
Another object of the invention is to provide a method using an adjustable pressure relief valve that can be set at a predetermined value for accommodating molten resins having different flow characteristics.
It is another object of the invention to provide a method using an alternate flow path for accommodating excess molten resin produced by excess resin pressure.
It is a feature of the invention that a method for releasing pressure uses a pressure relief valve associated with the molding apparatus for providing excess molten resin access to the alternate molten resin flow passage when the pressure in the molten resin passage is beyond a predetermined value.
To achieve one or more of the above objects, features and advantages of the invention, there is provided in one aspect of the invention, a method of releasing pressure in an injection mold having a non-metallic mold and a first, molten resin flow path for receiving a molten resin, comprising the steps of:
providing a pressure relief valve having an outwardly extending, axially movable, biased pin member;
forming a second molten resin flow path in an adjacent structure associated with said pressure relief valve for excess resin flow away from said non-metallic mold for use when pressure in said first molten resin flow path exceeds a predetermined value, said alternate molten resin flow path being configurable for fluid communications with said first molten resin flow path;
positioning said axially movable, biased pin member at a terminal end of the first molten resin flow path so as to block the flow of resin beyond said terminal end; and,
ejecting a molten resin through said molten resin flow path of said non-metallic mold such that any excess molten resin in said molten resin flow path creates excess pressure on said pin member of said pressure relief valve thereby causing said biased pin member to retract away from said terminal end of said first molten resin flow path and then open said excess molten resin to fluid communications with said alternate resin flow path.
The method of the invention, therefore, has numerous advantageous effects over currently existing developments, including: parts can be molded in an epoxy injection mold without the prospects of mold failure; the pressure relief valve is adjustable for different resins (for instance PC requires a heavier spring and less adjustment pre-load than HIPS); the pressure relief valve is specific to the mold, i.e., once it is set up properly and adjusted for a specific resin, it operates to protect the mold in any molding machine that one might use; the pressure relief valve resets itself automatically after it actuates; and, any part molded when the pressure relief valve actuates will eject from the mold with the resin from the escape channel attached.